The semiconductor wafer is produced as a product through plural production processes. Among the production processes, a chemical mechanical polishing process polishes a wafer surface by a cloth (polishing cloth) which is contacted to it by a polishing apparatus while performing circulation supply of a polishing slurry thereto.
This slurry might contain an ionized metal such as copper, nickel, iron or the like in a mixed form. Namely, alkali is used as one of components of the polishing slurry. Especially, alkali, for example, sodium hydroxide is produced by electrolysis of salt, and the produced sodium hydroxide contains various types of metal impurities of the order of several ppm.
In the chemical mechanical polishing process on a wafer having a copper film formed on it, copper is dispersed into the slurry and partly ionized by polishing.
Metals such as calcium, magnesium, manganese, iron, cobalt, zinc, aluminum and lead are hardly dispersed into the silicon wafer but remain as residues on its surface.
Among the metal impurities, for example, copper and nickel penetrate into the silicon wafer to remain in it and change electric properties to inhibit flattening of the surface. When the metals such as copper and nickel which easily diffuse into the wafer and other metals which remain on the wafer surface are considered for their influences depending on their concentrations, the concentrations of the metals which easily diffuse must be sometimes reduced to 1/10 to 1/1000 in comparison with those which remain on the surface. Therefore, when the slurry containing metal impurities of copper, nickel and the like is used to polish the wafer, there is a problem that a yield is decreased because the metals diffuse into the wafer and then are deposited on the wafer surface by heat treatment of the wafer performed later, a long-time storage, or the like.
Impurity metals such as calcium, magnesium, manganese, iron, cobalt, zinc, aluminum and lead also inhibit electric properties and others, so that if such impurity metals are contained in a polishing liquid in the chemical mechanical polishing process, they become loads in the subsequent washing process.
To deal with the above problems, there is proposed a chelating agent adding method by which a chelating agent is added to a slurry to form a metal complex (see, for example, JP-A 63-272460 (KOKAI)).
According to the chelating agent adding method, the chelating agent added to the polishing slurry is bonded to the metal to form a metal complex, and the metal complex electrically repels from the wafer to prevent metal contamination.
But, if the metal complex remains on the wafer surface, electric properties and others are also inhibited, so that if it is contained in the chemical or the polishing liquid in the alkali etching process or the polishing process, it increases the load in the subsequent washing process.
An amount of metal contamination decreases as the added amount of the chelating agent increases, but if the added amount of the chelating agent becomes large, a polishing rate of the wafer decreases, and therefore the added amount of the chelating agent is desirably as small as possible (see, for example, JP-A 2005-103700 (KOKAI)).    [Reference 1] JP-A 63-272460 (KOKAI)    [Reference 2] JP-A 2005-103700 (KOKAI)